This invention relates to computer disk substrates and, more particularly, to aluminum disk substrates prepared from aluminum powder compositions.
The term "powder metallurgy" generally characterizes a process for making a wide range of components and shapes from a variety of metals and alloys in powder form. The process is automated and uses pressure and heat to convert the powder into metal parts of net or near-net shapes, which require a minimum amount of secondary finishing.
The most common metals available in powder form are iron, tin, nickel, copper, aluminum and titanium, and refractory metals such as tungsten, molybdenum, tantalum and niobium. Prealloyed powders such as low-alloy steels, bronze, brass and stainless steel are produced in which each particle is an alloy. Nickel-cobalt-base superalloys and tooled steels are also available in powder form. Metal powders are highly engineered materials that range in size from 0.1 to 1,000 micromicrons with shapes that are spherical, acicular, irregular, dendritic, straight, angular or fragmented. Particle shape has a direct influence on the density, surface area, permeability and flow characteristics of a powder. Porosity of the powder particle varies with the method of production and influences the density of the particle and the final product.
Metal powders are produced by three major methods which can be classified as physical, chemical and mechanical. The most important physical method of metal powder production is atomization, wherein a stream of molten metal is broken up into droplets that freeze into powder particles. In most atomizing processes, a gas or a stream of liquid such as water impinges upon the liquid metal stream to break it into droplets. Iron, steel, aluminum, copper, stainless steel, brass, tin, bronze, zinc, and high-alloy powders are made in this manner. In the chemical method of powder production, a metal powder is produced by chemical decomposition of a compound of the metal as illustrated by the reduction of copper oxide to copper powder with hydrogen and by the electrodeposition of copper powder from an aqueous copper sulfate solution. In the mechanical method, certain materials such as iron, iron-aluminum alloys, ferrosilicon and ferrophosphorous can be converted into powder form by mechanical comminution.
In the basic powder metalurgy process, intense pressure ranging from about 10 tons per square inch to in excess of 60 tons per square inch are applied to the powder at ambient temperatures to obtain a green compact which is heated and sintered in a suitable furnace to bond the particles into a strong configuration which is observed as an increase in mechanical and physical properties.
Although most powder metallurgy parts and products are pressed in mechanical or hydraulic compacting presses, other techniques such as cold and hot isostatic pressing, powder metallurqy forging, and direct powder rolling are used to make a variety of end products. In isostatic compacting, gas or fluid pressure is applied uniformly to a flexible mold or container holding the metal powder to be compacted and since pressure from all directions is applied to the powder mass, it is possible to obtain a very uniform unsintered density and high uniformity of properties. In hot isostatic pressing, metal powder is placed in a sealed mold and then subjected to isostatic pressure at elevated temperatures. Powder metallurgy forging provides parts which are stronger than conventional powder metallurgy parts and provides tolerances and finishes that, in most cases, require little or no subsequent machining. In the forging process, preforms are compacted in a conventional compacting press, sintered, fed through a heating unit at a selected temperature, and transferred into the die area and formed in a closed die to its final shape with one cycle of the press; the formed part is then subjected to final heat treatment and machining, if required. In powder rolling, sometimes called roll compacting, metal powder is fed from a hopper into the gap of a rolling mill and emerges as a continuous compacted strip or sheet. Coinage strip is made by powder rolling.
Various additives and processing techniques have been disclosed in the patent literature for use in converting metal powder compositions to solidified finished forms by powder metallurgy.
U.S. Pat. No. 2,744,011 (Samuel et al., 1956) discloses a powder metallurgy process which comprises admixing a sinterable metal powder such as iron powder with a volatile liquid carrier, 4 such as methanol, containing a flux, such as ethyl silicate, to obtain a paste, forming the paste into an article, drying the article and subjecting the article to sintering treatment at, for example, 1380.degree. C. for 2 hours. It is pointed out that the flux acts as a bonding agent for the particles of the powder in the cold and, during sintering, as a solvent for films of oxide and other compounds on the surface of the particles, as well as a protective agent against further oxidation, with the absence of flux preventing contact of free particles and satisfactory interpenetration. It is further noted that boric acid and borates are substances whose action is primarily one of fluxing during sintering but these substances, in contrast to silicates and silicones, do not set hard from the liquid state and cannot, therefore, be used alone for compacting and agglomerating the powder.
U.S. Pat. No. 3,166,833 (Globus, 1965) discloses a powder metallurgy process which comprises admixing and milling a metal powder, such as titanium powder, with an alkali metal borohydride, such as sodium borohydride, compacting the admixture at a pressure between 10 and 50 tons per square inch to produce green briquettes having a density of about 60 to 80% of the theoretical density, and sintering in a high vacuum furnace for about 4 hours or more at a temperature between 2,000.degree. and 2,500.degree. F. and a vacuum of 1 to 2 microns of mercury. It is suggested that the alkali metal borohydride decomposes and reacts during the sintering, liberating the alkali metal, which combines with and removes the oxygen and thus acts as a deoxidizing agent while the boron combines with the metal to form a stable boride.
U.S. Pat. No. 3,232,753 (St. Pierre, 1966) discloses a powder metallurgy process which comprises milling aluminum powder in an aqueous solution of copper sulfate to remove the oxide coating from the aluminum powder and coat the aluminum powder with copper, separating the copper coated aluminum powder from the solution, compacting the coated aluminum powder and sintering the compact in a non-oxidizing atmosphere.
U.S. Pat. No. 3,250,838 (Bartoszak, 1966) discloses a powder metallurgy process which comprises blending aluminum powder, copper flake, copper powder and a stearate lubricant, compacting the resulting mixture to obtain green compacts, and subjecting the compacts to sintering.
U.S. Pat. No. 3,687,657 (Storchheim, 1972) discloses a powder metallurgy process which comprises mechanically mixing aluminum powder, copper powder and magnesium or zinc powder, compacting the resulting mixture in a die coated with a lubricant to a density between 92 and 97% of theoretical density, and selectively heating-up the resulting compact to sintering temperature in ambient atmospheric air.